1. Field of the Invention
The present invention relates to a gas sensor disposed in an intake passage, such as an intake gas passage through which intake gas drawn into an internal combustion engine from the outside flows, or an intake recirculation gas passage through which intake recirculation gas, which is exhaust gas recirculated to reduce the exhaust of pollutants from an internal combustion engine, flows.
2. Description of the Related Art
A technique is known in which a gas sensor is disposed at an intake gas passage in an internal combustion engine to detect the oxygen concentration of intake gas (atmosphere) drawn into the internal combustion engine from the outside. The detected oxygen concentration is used for the operation control of the internal combustion engine, thereby improving the precision and the like of the air-fuel ratio control of the internal combustion engine (see, for example, Patent Document 1).
In addition, in recent years, a technique in which exhaust gas is reintroduced into an intake system in order to reduce nitrogen oxides (NOx) exhausted from an internal combustion engine (hereinafter referred to as “exhaust gas recirculation” or “EGR system”) is known (for example, refer to Patent Document 2). Providing a gas sensor at an intake recirculation gas passage in order to detect the oxygen concentration of intake recirculation gas, which is a mixture of the exhaust gas and intake gas of the EGR system, is also known.
As a gas sensor mounted in an intake passage, such as an intake gas passage or an intake recirculation gas passage, for example, a gas sensor having a detecting element in which the intensity of the electromotive force or the resistance value varies with the concentration of NOx (nitrogen oxides), oxygen, and the like is known (for example, refer to Patent Document 3). Since this gas sensor is exposed to high-temperature gas, such as intake gas or intake recirculation gas, and moisture included in the gas attaches to the gas sensor (the gas sensor is covered with water), there is a concern that the detecting element may be subjected to thermal shock, and cracking or fracturing may occur. Therefore, a protector that covers the detecting element is mounted in the gas sensor so as to protect the detecting element from being covered with water. Furthermore, while gas flowing toward the intake passage side includes more soot (carbon) than gas flowing toward the exhaust passage, the protector can also prevent the soot from attaching to the detecting element, and can prevent the detecting precision of the detecting element from being degraded.
In addition, under the assumption that the protector has a dual structure of an inside protector and an outside protector, a gas sensor having an external gas introducing portion for introducing intake gas to the inside of the protector provided on the side face of the tube-shaped portion of the outside protector and an external gas releasing portion for releasing the intake gas from the inside of the protector disposed at the bottom portion (leading end face) of the outside protector is known (see, for example, Patent Document 4). Thereby, even when the external gas introducing portion is made relatively large in consideration of clogging in the external gas introducing portion, soot attaches to the inner wall portion of the inside protector, and attachment of soot to the detecting element can be suppressed. As a result, clogging of the external gas introducing portion can be suppressed (degradation of the detecting precision of the detecting element is suppressed), and the occurrence of fracturing or cracking in the detecting element can also be suppressed.